The present invention relates to an air bag system installed in a motor vehicle for protecting the driver and passenger in case of a front-end crash of the vehicle, and more particularly to a circuit for operating the air bag system.
An air bag is folded and packed in a housing mounted on a steering wheel or on a dash board of the motor vehicle. During a crash, the air bag inflates in a fraction of a second in accordance with a signal from a g-responsive sensor or a deceleratometer so as to protect the driver and passenger. In order to inflate the air bag, an inflator is provided on the steering wheel. The inflator has a squib as a heating element, and an air bag operating circuit including a source, a g-responsive sensor, and a connector for electrically connecting these components to each other. At a crash, a current from the source is supplied to the squib to heat gas in the inflator to explode it, so that the air bag is inflated.
FIG. 2 shows a conventional air bag operating circuit. An inflator 1 is provided on a steering wheel. Further, a horn switch 2, and a cruise control switch comprising a set switch 3a and a resume switch 3b are provided on the steering wheel.
On a vehicle body, a pair of first g-responsive sensors 5a and 5b are provided in the inside of the vehicle and a pair of second g-responsive sensors 6a and 6b are provided on side frames in a front portion of the vehicle body for detecting a crash of the vehicle. A horn 8, a relay coil 7a and a relay switch 7b and a cruise control unit 9 are further provided. A connector 25 is provided on the steering wheel for electrically connecting the components on the steering wheel and vehicle body. The connector 25 comprises a reel having a case 25a secured to a fixed portion such as a steering column and a shaft 25b secured to a steering wheel. A flat cable having a plurality of leads is loosely wound by few turns on the shaft 25b. An end of the inflator 1 is connected to a lead 10a of the cable in the connector 25, and further connected to a source B1 through sensors 5a and 5b. The other end of the inflator 1 is connected to the ground through a lead 10b and sensors 6a and 6b. The horn switch 2 is connected to the ground at an end thereof and the other end of the switch 2 is connected to another source B2 through a lead 10c and the relay coil 7a. The relay switch 7b is connected to the source B2 and to the horn 8. The set switch 3a and the resume switch 3b are parallely arranged each other and connected to the lead 10c in series and to the cruise control unit 9 through leads 10d and 10e. Input terminals of the cruise control unit 9 are connected to the ground through respective resistors having comparatively large values (2K .OMEGA.). Thus, when one of the switches 3a and 3b is closed, the current flowing the relay coil 7a reduces, thereby preventing the switch 7b from closing.
The conventional air bag system comprises a single circuit including the sensors 5a, 5b, lead 10a, inflator 1, lead 10b, and sensors 6a, 6b. If the circuit is disconnected, the current does not flow in the inflator 1, so that the air bag may not operate.
In order to solve such a serious problem, it may be proposed that a circuit is connected to the single air bag circuit in parallel. Such an addition of the circuit increases the number of leads from five to seven. However, since the space in the reel is small, it is difficult to house the increased lead in the case 25a.